Coordinators of national monitoring schemes deliver their national data to the PECBMS coordination unit annually. For information on data and contributing countries see Box Participating countries.

The data delivered are: the national yearly indices per species, the all-sites yearly totals (= the sum of birds counted across all sites per year) and their standard errors, and the covariances between the yearly figures.
Specifically, national coordinators deliver two files per species – so called out and ocv files (see paper by Pannekoek & Van Strien, 2001 or also see website of Statistics Netherlands) – both produced by TRIM when calculating species indices at the national level. These TRIM output files are accompanied by a species list and by comments indicating any potential problems in the data. These comments are taken into account in further data analysis.
National data are checked for their quality using quantitative criteria (see Box Data checks).

Box Data checks

After receipt, the PECBMS coordination unit checks the national data on species numbers, indices and trends for inconsistencies; it checks for instance if the scheme all-sites totals exceed the national population size known from other sources. Suspicious data coming from countries with more than 10% of PEBCMS European bird populations (see below) are examined in detail. If any problems are encountered, the coordinators of national monitoring schemes are consulted. Additionally, documentation from previous data analysis is checked and used for decisions in some cases. Data with persistent serious data problems are excluded from computation.

Suspicious data – national species data are subject to closer examination when:

Slope (Multiplicative) < 0.6

Slope (Multiplicative) > 1.5

Slope (Multiplicative) standard errors > 0.5

Percentage of scheme time totals of the species > 95% of national population size of the species in European Red List of Birds (BirdLife International 2015)

Ratio of national population size to scheme time totals > maximum of species population size in European Red List of Birds (BirdLife International 2015)

Number of zero counts < 1

Number of missing counts < 1

Index value < 0.5

Index value > 1000

Scheme time totals < 1

Scheme time totals > 1000000

More than one year with index = 100 and SE = 0 present in the results

For these data checks an automatic system has been developed by Statistics Netherlands.

All national indices – species by species – are also checked for their interannual consistency (comparison with previous trends and indices) and all suspicious and inexplicable inconsistencies in indices are examined in detail. Also for this procedure a special tool has been developed.

Box Participating countries

Monitoring data available to PECBMS come from more than 20 countries. The number of countries contributing data and the number of species covered have increased almost every year, as new schemes have started up and provided their data. Almost all EU Member States are now represented, along with some neighbouring countries.

Number of countries contributing to the PECBMS has increased since 2003 from 18 to 28. Moreover, some of the countries with a scheme established recently have not contributed their data to PECBMS yet, but are expected to do this in near future. The overall number of species for that the PECBMS produces trends and indices has almost tripled since 2003 (see graph below).

For details and updated information on national monitoring schemes browse country reports.

2.2. Combining national data into supranational
outputs

A method has been developed to produce supranational yearly totals and their standard errors across countries by combining the national data. The method takes into account the differences in population sizes per country, as well as the differences in field methods and in the numbers of sites and years covered by the national schemes. Instead of deriving the standard errors in the usual statistical way from count data and model fit, standard errors (and the year-year covariances) that resulted from the calculation of the all-sites totals per country were applied. The results are similar to those that would be produced when the raw data are being used (Van Strien et al, 2001).

To produce supranational indices, the national all-sites totals per species as assessed in the national monitoring schemes are combined. A weighting factor is introduced to adjust for differences in national population sizes, to make sure that a change in a larger national population has an accordingly greater impact on the overall trend than a change in a smaller population. The weighting factor is calculated as the national population size derived from BirdLife International (2015) in 2008-2012 divided by the average of the all-sites totals for 2009-2011. This weighting factor is applied to all other years of the scheme. By this weighting, the yearly scheme totals are converted into yearly national population sizes.

The national European monitoring schemes have started in different years, leading to missing national all-sites totals. An adapted version of TRIM is used to estimate the missing country totals, in a way equivalent to imputing missing counts for particular sites within a country. After these weightings and imputation steps, the national totals are summed up to European totals.

European species indices for a species are computed if data are available from countries which together host at least 50% of the ´PECBMS European´ population of that species. ´PECBMS Europe´ is EU 27 + Norway and Switzerland and consists of those countries which already deliver their data to PECBMS or are supposed to do so in the near future: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Netherlands, Norway, Poland, Portugal, Republic of Ireland, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Kingdom.

However, some parts of these countries (states) were excluded, mainly because of their far distance to the mainland of Europe: Faroe Islands and Greenland, Svalbard, Azores, Madeira, Canary Islands, Gibraltar.

Population species trends (multiplicative slopes) are computed and classified in the same way as at the national level.

Box Missing values II

Ideally, in a European breeding bird monitoring scheme all countries start in the same year. If so, it is relatively easy to assess the changes in the yearly all-countries totals of breeding pairs.

The procedure then is as follows. First the national yearly all-sites totals are converted into national yearly total population sizes in that country. To do so, information of total population sizes collected by BirdLife International (2015) is used. A weighting factor is calculated by dividing the population total as assessed by BirdLife International (2015) by the estimated all-sites totals for those years that are covered by population size estimates (BirdLife International 2015) and a monitoring scheme. Subsequently, this weighting is applied to all years of the monitoring scheme, so that the weighted year totals may be considered as the yearly population totals in that country. An example may clarify this. Suppose the estimated all-sites total of species X in the UK amounts 100 in the year 2009 and 110 in 2010. If the population total for the UK would be 1000 in the year 2009, the weighting factor equals 1000/100 = 10. For the year 2010, the population total is 10*110 = 1100.

The standard error of the yearly population totals are (weight factor) x (standard error of all-sites total).

Subsequently, the population totals for each country are summed to yield the supranational population totals for each year. The standard errors of the supranational totals were derived by the following statistical rule: variance of supranational total = variance of country1 total + variance of country2 total + variance of country3 total etc., where variance = standard error2. This rule applies because the estimates of the yearly totals are independent between countries. Finally, the supranational totals are converted into indices.

The national European monitoring schemes, however, started in different years, leading to missing national all-sites totals. Just as was explained in the example for sites above, simple comparisons of the yearly sum of country totals will then give misleading inferences on trends, because lesser countries contribute in earlier years. Again we used an adapted version of TRIM to estimate the missing country totals, in a way equivalent to imputing missing counts for particular sites. Basically we computed indices for four European regions (West Europe, North Europe, Central & East Europe and South Europe) and derived missing national totals from changes in countries in the same European region, as the example shows.

year 1

year 2

year 3

Country 1 population total

4000

3000

2000

Country 2 population total

4000

3000

2000

Country 3 population total

missing

missing

8000

etc.

West Europe population total

24

18

12

Thereafter, the regional totals were calculated and summed to obtain European indices as described above.

An extra complication is that also regions differ in the years covered. For instance, the first country in South Europe started its monitoring activities only in 1989, whereas monitoring started in 1982 in Central & East Europe and even earlier in West Europe. To combine totals from the different regions, we performed a third step to combine results using TRIM. All-together we applied a refined hierarchical imputation procedure to combine country population totals.

To produce supranational indices, we used a slightly adapted version of TRIM tailored to combining all-sites totals and their standard errors instead of raw counts per site. Instead of deriving the standard errors in the usual statistical way from count data and model fit, we applied the standard errors (and the year-year covariances) that resulted from the calculation of the all-sites totals per country.

In summary, PECBMS combines data from national monitoring schemes which differ in several aspects. These differences are addressed in order to produce unbiased results with known precision (Van Strien et al., 2001).

Overview of effects of methodological differences in national monitoring schemes:

Difference between countries (national schemes)

Influence on national indices

Consequences for supranational indices

Field method

Precision

Include standard error

Number of sites

Precision

Include standard error

Site selection method

Bias

Remove bias at national level

Index method

Bias/precision

Use proper method (TRIM)

Years covered

Missing yearly indices

Estimate missing indices

Population size

None

Weight indices by national population size

When supranational indices and trends are produced, a control for interannual consistency is carried out just as was done for national data (see details in chapter 4. Quality control). In case some inconsistencies had occurred, they are examined in details to seek whether the inconsistency is caused by enlarged or improved data set or by computation errors.

Species indices and trends are produced for Europe and its regions (Central & East, North, South, West, and Southeast Europe, East Mediterranean and West Balkan) and for EU and its regions: New (since 2004) and Old EU Members States).

The species indices are always computed for maximum time period given by the country with the longest data set within the region (e.g. indices for West Europe are produced since 1966 given by the starting year of the indices from UK, while indices from South Europe are produced since 1989 given by the starting year of indices from France – see an overview below).

Overview showing list of 28 countries, their start years and region that each country belongs to:

Country/region

Region(group of countries)

First year

Last year

Austria

WE

1998

2016

Belgium-Brussels1)

WE

1992

2016

Belgium-Wallonia1)

WE

1990

2016

Bulgaria

SEE

2005

2016

Cyprus2)

East Mediterranean

2006

2016

Czech Republic

CEE

1982

2016

Denmark

WE

1976

2016

Estonia

CEE

1983

2016

Finland

NE

1975

2016

France3)

SE

1989

2016

Germany East4)

CEE

1991

2016

Germany West4)

WE

1989

2016

Greece

SEE

2007

2016

Hungary

CEE

1999

2016

Italy

SE

2000

2015

Latvia5)

CEE

1995

2016

Lithuania

CEE

2011

2016

Luxembourg

WE

2009

2012

Netherlands

WE

1984

2016

Norway6)

NE

1996

2016

Poland

CEE

2000

2016

Portugal

SE

2004

2014

Republic of Ireland

WE

1998

2016

Romania

SEE

2007

2016

Slovakia

CEE

2005

2016

Slovenia

West Balkan

2008

2016

Spain

SE

1998

2016

Sweden7)

NE

1975

2016

Switzerland

WE

1999

2016

United Kingdom

WE

1966

2016

Explanations:WE – West EuropeNE – North EuropeSE – South EuropeSEE – Southeast EuropeCEE – Central & East EuropeFirst year – first year of data time series in a country/regionLast year – last year of data time series in a country/region
Time series for individual species from national schemes are shorter in certain cases.

Countries notes:1) Data for Belgium were combined from Wallonia and Brussels regions.2) Data for Cyprus come from two schemes that partly differ in their regional coverage, Volunteer Common Birds Census (2006-2016) and Western Cyprus Common Bird Census (2006-2011). Data from both schemes were combined.3) Data for France come from two schemes, old (1989-2001) and new one (2001-2016). Data from both schemes were combined but not updated this year.4) Data for Germany were combined from schemes in former East and West Germany and also newly from old scheme (Häufige Brutvögel alt, 1989-2010) and new scheme (Häufige Brutvögel neu, 2005-2016).5) Data for Latvia come from three different schemes, two old ones (differ in their regional coverage, and cover the periods 1995-2006 and 2003-2006, respectively) and a new one (2005-2016). Data from all schemes were combined.6) Data for Norway come from three schemes, Norsk Hekkefugltaksering, HFT (1996-2008), Terrestrisk overvåking, TOV-I (1996-2008) and Terrestrisk overvåking – Ekstensiv, TOV-E (2006-2016). Data from all schemes were combined.7) Data for Sweden come from two schemes, old (1975-2015) and new one (1998-2016). Data from both schemes were combined.

Although we produce all species indices for the maximum time period (since the first year of time period available), we publish only the European indices since the year 1980 at the earliest since when the index is based on data from several countries – see the latest version of species indices and trends.

The species trends are produced for the maximum time period and for shorter periods as well: since 1980, 1990, 2000, 2007 and 2012. For simplicity only, we publish the long-term trends (the trends which starting year varies from 1980 to 1998) and the ten-year trends (the trends for last ten years, i.e. 2007-2016).